An energy storage device, especially a lithium secondary battery, has been widely used recently for a power source of an electronic device, such as a mobile telephone, a notebook personal computer, etc., and a power source for an electric vehicle or electric power storage. There is a high possibility that a battery mounted on such an electronic device or a vehicle is used at a high temperature in midsummer or in the environment warmed by the heat generation of the electronic device.
A lithium secondary battery is mainly constituted of a positive electrode and a negative electrode, each containing a material capable of absorbing and releasing lithium, and a nonaqueous electrolytic solution composed of a lithium salt and a nonaqueous solvent, and a carbonate, such as ethylene carbonate (EC), propylene carbonate (PC), etc., is used as the nonaqueous solvent.
In addition, a lithium metal, a metal compound capable of absorbing and releasing lithium (e.g., a metal elemental substance, a metal oxide, an alloy with lithium, etc.), and a carbon material are known as the negative electrode of the lithium secondary battery. In particular, a nonaqueous electrolytic solution secondary battery using, as the carbon material, a carbon material capable of absorbing and releasing lithium, for example, coke or graphite (e.g., artificial graphite or natural graphite), etc., is widely put into practical use.
Since the aforementioned negative electrode material stores and releases lithium and an electron at an extremely electronegative potential equal to the lithium metal, it has a possibility that a lot of solvents are subjected to reductive decomposition particularly at a high temperature, and a part of the solvent in the electrolytic solution is reductively decomposed on the negative electrode regardless of the kind of the negative electrode material, so that there were involved such problems that the movement of a lithium ion is disturbed due to deposition of a decomposed product or generation of a gas, thereby worsening battery characteristics, such as cycle properties particularly at a high temperature, etc., and further worsening heat stability of the negative electrode. Furthermore, it is known that a lithium secondary battery using a lithium metal or an alloy thereof, a metal elemental substance, such as tin, silicon, etc., or a metal oxide thereof as the negative electrode material may have a high initial battery capacity, but the battery capacity and the battery performance thereof, such as cycle properties, may be largely worsened particularly at a high temperature because the micronized powdering of the material may be promoted during cycles, which brings about accelerated reductive decomposition of the nonaqueous solvent, as compared with the negative electrode formed of a carbon material.
Meanwhile, since a material capable of absorbing and releasing lithium, which is used as a positive electrode material, such as LiCoO2, LiMn2O4, LiNiO2, LiFePO4, etc., stores and releases lithium and an electron at an electropositive voltage of 3.5 V or more on the lithium basis, it is known that in an interface between the positive electrode material and the nonaqueous electrolytic solution, a decomposed product or a gas generated by local oxidative decomposition disturbs a desirable electrochemical reaction. There is a possibility that a lot of solvents are subjected to oxidative decomposition particularly at a high temperature, and a part of the solvent in the electrolytic solution is oxidatively decomposed on the positive electrode regardless of the kind of the positive electrode material, so that there was involved such a problem that the movement of a lithium ion is disturbed due to deposition of a decomposed product or generation of a gas, thereby worsening battery characteristics, such as cycle properties, etc.
Irrespective of the foregoing situation, the multifunctionality of electronic devices on which lithium secondary batteries are mounted is more and more advanced, and the electric power consumption tends to increase. The capacity of the lithium secondary battery is thus being much increased, and shortening of a charging time is demanded, too. But, in the case of repeating the charging and discharging cycle at such a high load, an absorbing reaction of a lithium ion in the negative electrode does not uniformly occur over the entirety of the negative electrode, and metallic lithium is apt to deposit on the negative electrode surface where the reaction is concentrated, whereby heat stability of the negative electrode is worsened, and also, a decomposition reaction of the electrolytic solution proceeds. For this reason, it is demanded to improve high-load charging and discharging cycle properties, heat stability of the negative electrode, and safety.
With respect to the safety, PTL 1 discloses a nonaqueous electrolytic solution composed of a nonaqueous solvent including a phosphoric acid ester compound, such as triethyl phosphonoacetate, etc., and an electrolyte and describes that the electrolytic solution exhibits self-fire extinguishing property.
In addition, PTL 2 discloses a nonaqueous electrolytic solution containing, as an additive, a phosphoric acid ester compound, such as triethyl phosphonoacetate, triethyl phosphonoformate, etc. and describes that the continuous charging characteristics and high-temperature storage properties are improved, and the gas generation can be suppressed.
PTL 3 describes that a lithium secondary battery using a nonaqueous electrolytic solution including a phosphonoacetate compound, such as triethyl phosphonoacetate, etc., is capable of exhibiting high-temperature storage properties or suppressing swelling of the battery.